The initial objective of the proposed research is ultrastructural characterization of a murine Leydig cell tumor known to respond to administered gonadotropin with a rapid increase in testosterone production. Electron microscopy will be used to classify the cell types present in the tumor and to explore their patterns of intracellular organization. Ultrastructural changes that accompany the functional response of the tumor cells to gonadotropin will be defined in relation to their sequence of development. Particular attention will be directed to utilization of the tumor cells as an experimental model for studies of the detailed relationships of intracellular structures to steroid hormone production. This phase of the research involves incubation of tumor cells in media containing gonadotropin in combination with a variety of agents known to interfere with cholesterol metabolism. Measurements of esterified cholesterol depletion in the cells and of testosterone secretion into the medium will be correlated with ultrastructural observations of the incubated cells. Specific issues to be resolved include 1) the structural manifestation of the hormone-precursor pool, 2) the relationship of mitochondrial structure to hormone synthesis, 3) the significance of various structural forms of smooth endoplasmic reticulum in steroid production, and 4) ultrastructural events in steroid hormone secretion. During maintenance of the tumor in our mouse colony, the influence of administered gonadotropin on tumor growth will be investigated. The relationship between uncontrolled growth, which may be inhibited by gonadotropin, and functional differentiation, which is enhanced by gonadotropin, requires clarification. The relative contributions of growth and function to the virulency of the tumor will be explored.